1. Field of the Invention
The present invention relate to a technology for demodulating servo-information read from a storage medium of a storage device via a head.
2. Description of the Related Art
Recently, recording density of storage devices such as a magnetic disk device, a magneto-optical disk device, and an optical disk device has been dramatically improved. With the improvement of the recording density, it is possible to downsize storage devices or increase storage capacities, which makes it easier to use the storage devices.
In a storage medium with a high recording density, an automatic control, called a servo control, is employed to operate a head to accurately position at a target track. The storage medium has a signal pattern written thereto. The signal pattern, called servo information (servo data), is used for servo control. The servo information contains information on at least a track number and a servo pattern used for demodulating a relative position of the head to a center of a track.
To operate the head to position at the center of the target track exactly, a velocity control of the head is essential as well as a positional control in which the head is operated accurately based on positional information indicating the relative position of the head to the center of the track obtained from the demodulated servo pattern. By using the servo pattern of servo information, it is possible to demodulate a detailed position of the head and a moving velocity of the head.
By controlling a current supplied to a voice coil motor for driving the head to travel based on a difference between a moving velocity of the head at the current tract and a moving velocity of the head to a target track, it is possible to operate the head to position at the target track rapidly and accurately.
However, because the demodulated moving velocity of the head is a value obtained from an estimation based on the positional information of the head demodulated from the servo information, the demodulated moving velocity may include an error and may differ from a real moving velocity.
Japanese Patent Application Laid-open No. H5-159488 discloses a device that controls velocity of a head by demodulating positional information of the head from demodulated servo information. The device is capable of correcting a range of a moving distance between a current track, at which the head is positioning, and a target track, to which the head moves, based on a previously calculated moving distance to the target track, and calculating a moving velocity of the head at the current track more correctly.
Servo-information demodulating methods in which the moving velocity is demodulated together with the positional information of the head from a demodulated signal read from the servo information are disclosed in Japanese Patent Application Laid-open No. H8-180620, Japanese Patent Application Laid-open No. H2-168411, Japanese Patent Application Laid-open No. 2000-123506, Japanese Patent Application Laid-open No. 2003-338145, Japanese Patent Application Laid-open No. 2003-346438, and Japanese Patent Application Laid-open No. 2000-173210. According to these methods, the moving velocity of the head is directly demodulated from the servo information instead of the demodulated positional information. Therefore, it is possible to demodulate a more correct moving velocity.
However, according to the above conventional technique disclosed in Japanese Patent Application Laid-open No. H5-159488, a moving velocity of the head is calculated from a moving distance of the head to a target track that is corrected based on positional information of the head obtained from demodulated servo information. Therefore, the moving velocity of the head becomes demodulated from a mere estimation, which leads to a limitation in demodulating a more correct moving velocity.
In the above conventional techniques disclosed in the other literatures, it is not considered occurrence of an overflow associated with the demodulated velocity of the head caused by an increased speed of a seek operation. The demodulated moving velocity is correct without the overflow. However, if the overflow occurs, it is hard to demodulate a correct moving velocity.